The present invention relates generally to an optical fiber for multi-wavelength transmission. Moreover, the present invention relates to an apparatus and method for reducing the total attenuation and non-linear effects associated with long distance optical transmission.
In today's worldwide communication systems, it is often necessary to extend a transmission line over a long distance, which may include a body of water, to provide a communication link between a transmitter and a receiver. The current trend in communication systems is to use optical fibers to make these transmission lines. Optical fibers are preferred because the fibers can transmit a large number of digital signals at a high data transmission rate.
To further improve the signal carrying capacity of the transmission line, optical fibers can be used with Wavelength Division Multiplexing (WDM) technology. This technology allows multiple optical signals to be sent through the same fiber at closely spaced wavelength channels. This greatly enhances the information carrying capacity of the overall transmission system.
Several problems are encountered when optical fibers are used to transmit signals over a significant distance. For example, the power of the optical signal decreases as the signal travels through each fiber. This power loss, also called attenuation, can be compensated for by including amplifiers along the transmission line to boost the power of the signal. The placement and number of amplifiers along the transmission line is partly determined by the attenuation of the optical fiber. Obviously, a signal sent through a fiber with a low attenuation needs fewer amplifiers than a signal sent over a fiber with a high attenuation.
Chromatic dispersion is another problem encountered when transmitting signals over optical fibers. Chromatic dispersion, hereafter referred to as “dispersion,” arises from the optical fiber transmitting the different spectral components of an optical pulse at different speeds, which can lead to the spreading or broadening of an optical pulse as it travels down the transmission line. Each optical fiber has a dispersion value that varies as a function of the wavelength of the optical signal and arises from the material composition of the glass optical fiber and the waveguide characteristics. The dispersion within the optical fiber at a given wavelength can be positive, negative, or zero, depending on the transmission characteristics of the fiber. Despite the type of dispersion (positive or negative), excessive amounts can lead to detection errors at the receiver of the optical signal.
Transmitting signals at the zero-dispersion wavelength of a fiber will practically eliminate the dispersion problem, but can exacerbate other transmission problems, particularly non-linear effects when used with WDM systems. A particularly relevant non-linear effect in WDM systems is the phenomenon of Four Wave Mixing (FWM). FWM occurs when at least two signals verifying phase matching conditions are sent through the same fiber (as in WDM systems) and interact to generate new wavelengths. In the case of WDM systems having a large number (more than two) of equally spaced channels, these new wavelengths will eventually overlap with the signal wavelengths, thus degrading the Signal-to-Noise Ratio. It is known that WDM systems that have an operating wavelength different from the zero-dispersion wavelength of the transmission fiber (and therefore have a non-zero dispersion value at the operating wavelength) minimize FWM degradation. More precisely, FWM efficiency η, defined as the ratio of the FWM power to the per channel output power (assuming equal input power for all the channels) is approximately proportional to:
  η  ∝            [                                    n            2                    ⁢          α                                      A            eff                    ⁢                                    D              ⁡                              (                                  Δ                  ⁢                                                                          ⁢                  λ                                )                                      2                              ]        2  
where α is the fiber attenuation; n2 is the non-linear refractive index; Aeff is the fiber effective area; D is the dispersion; and Δλ is the channel spacing. The above approximation is valid under the condition α<<Δβ, where Δβ=(2πc/λ2)·D·Δλ2, c is the speed of light and λ the transmission wavelength. See D. W. Peckham, A. F. Judy and R. B. Kummer, ECOC '98, paper TuA06, pp. 139–140. As can be seen, for a given set of values for Δλ, n2 and α, to decrease FWM efficiency one can increase the absolute value of dispersion and/or increase the value of fiber effective area Aeff. On the other hand, decreasing channel spacing dramatically increases FWM efficiency.
Other non-linear effects include Self Phase Modulation, Cross Phase Modulation, Stimulated Brillouin Scattering (SBS), and Raman Scattering (SRS). It is well known that a fiber with a larger effective area at the operating wavelength is less susceptible to all non-linear effects.
To solve the dispersion and non-linear effects associated with sending signals through long optical fibers, conventional systems use transmission lines that connect spans of optical fiber that have alternating dispersion values. For example, a span of negative dispersion fiber can be followed with a span of positive dispersion fiber to even out the overall dispersion over the transmission line. This approach ensures that the dispersion is non-zero at local values throughout the transmission line to avoid non-linear effects and that the total dispersion over the cumulative transmission line is compensated to nearly zero at the receiver.
Various publications discuss different approaches to solve these problems. For example, U.S. Pat. No. 4,969,710 to Tick et al. discusses an optical fiber transmission path wherein total dispersion of the system is compensated by the use of fibers composed of glasses with total dispersion of opposite signs at the operating wavelength for the system.
U.S. Pat. No. 5,343,322 to Pirio et al. discusses a system for long distance transmission of a digital signal. The system uses optical fiber having a low negative dispersion to connect receiver stations that include dispersion compensation devices having positive dispersions to compensate for the negative dispersion.
U.S. Pat. No. 5,559,920 to Chraplyvy et al. discusses an optical communication system having an initial span of a strong negative dispersion followed by positive dispersion spans. The system overcompensates for the negative dispersion in that the final dispersion value is not zero.
Other publications, such as U.S. Pat. No. 5,587,830 to Chraplyvy et al., U.S. Pat. No. 5,719,696 to Chraplyvy et al., U.S. Pat. No. 5,675,429 to Henmi et al., and U.S. Pat. No. 5,778,128 to Wildeman also discuss transmission lines for long range systems. These publications disclose transmission lines that use varying combinations of fiber that have either a negative dispersion or a positive dispersion at the operating wavelength. The negative dispersion fiber and the positive dispersion fiber are arranged so that the total dispersion of the system is compensated to approximately zero.
Similarly, U.K Patent No. 2 268 018 also discusses an optical transmission system that combines optical fiber having a negative dispersion with fiber having positive dispersion to compensate the dispersion to zero for the total length of the transmission.
European Patent Application No. 0 790 510 A2 discusses a symmetric, dispersion-managed fiber optic cable. The cable of this disclosure includes a conventional single mode fiber having a positive dispersion at the operating wavelength connected to a second optical fiber that has a negative dispersion at the operating wavelength.
U.S. Pat. No. 5,611,016 to Fangmann et al. discloses a dispersion-balanced fiber optic cable. The cable includes single mode fibers having opposite dispersion at the operating wavelength. A WDM system interconnects two dispersion-balanced cables with a crossover connection between the positive-dispersion fiber of one cable and the negative-dispersion fiber of the other cable. According to this patent, improved performance is achieved when the absolute value of the chromatic dispersion of the single-mode fibers at a wavelength of 1550 nm is in the range 0.8–4.6 ps/nm/km.
S. Bigo et al. in the paper ‘1.5 Terabit/s WDM transmission of 150 channels at 10 Gbit/s over 4×100 km of TeraLight™ fibre’ PD2–9, pp. 40–41, ECOC'99 disclose a transmission line wherein four spans of 100 km of fiber having average chromatic dispersion of +8 ps/nm/km, a dispersion slope of 0.057 ps/nm2/km and an attenuation of 0.205 dB/km are interleaved with optical amplifiers including dispersion compensating fibers (DCF). Applicants have determined, based on the data provided in the paper, that the zero dispersion wavelength of the disclosed TeraLight™ fiber is lower than 1440 nm.
Applicants have noted that these prior arrangements use combinations of optical fiber that result in undesirably high levels of attenuation.
Applicants have faced the task of providing an optical fiber to be used in Dense and Hyper-Dense WDM transmission lines that reduces non-linear effects and at the same time minimizes attenuation and achieves a manageable dispersion.